The present invention relates to a method and apparatus for the polishing of turbine blades. The manufacture of turbine blades presents a difficult problem to the machinist. Turbine blades require a very complicated surface involving curves in three dimensions. The only known method of manufacturing a turbine blade in a single automated process is by precision forging. See, e.g., U.S. Pat. Nos. 4,526,747 and 4,489,469. However, precision forging of turbine blades is extremely expensive and is only practical when producing a large number of identical turbine blades. Moreover, creating the cast for precision forging is a time consuming and substantially manual procedure which suffers from the same drawbacks as the other methods of manufacturing turbine blades discussed below.
Known methods of polishing turbine blades include the electrical and chemical discharge methods. See, e.g., U.S. Pat. Nos. 4,234,397 and 4,141,127. These methods, like precision forging, are expensive, time consuming and only viable in the mass production of a large number of identical turbine blades.
Another known method of manufacturing turbine blades involves polishing a rough form turbine blade using a belt sander. See, e.g., U.S. Pat. Nos. 4,473,931; 4,285,108; and 3,925,937. In accordance with this method, a rough form for the turbine blade is manufactured by forging or milling within 5/1000 inch of the final form. The rough form is then polished by hand or mounted in a belt sanding machine tool which polishes the blade by removing no more than 5/1000 inch from the surface of the rough form. The blade must constantly be checked with guillotine guages during the sanding process and the measurements taken by the guages compared to a table of measurements to guarantee adequate conformity with the specific desired shape of the individual blade to be polished.
In order to appreciate the problems encountered with known methods and apparatuses for polishing turbine blades, it should be noted that the polishing of the turbine blade surface must be done in three dimensions. These dimensions may be viewed by reference to X, Y, and Z axes. Thus, the finished surface of the turbine blade is defined by at least two curves, one in the Z-Y plane and another in the Z-X plane. Typically, the surface of a turbine blade is more complex, being defined by a great number of curves in a corresponding number of planes parallel to the Z-Y plane and/or the Z-Y plane. (As used herein, Z-Y plane will mean the Z-Y plane and planes parallel to it. Similarly, Z-X plane will mean the Z-X plane and planes parallel to the Z-X plane. Z-Y curve will mean a curve in the Z-Y plane, and Z-X curve will mean a curve in the Z-X plane.)
Known methods and machine tools for polishing semi-finished turbine blades utilize a belt sanding device wherein a narrow sanding belt is arranged on a pulley known as a shoe. The axis of the shoe is usually arranged parallel to the longitudinal or X axis of the turbine blade. A turbine blade in rough form is mounted on a movable platform beneath the sanding belt. The sanding belt is brought into contact with the surface of the blade to be polished and the blade, via the movable platform, is moved along the X axis with respect to the shoe. While the blade is moving along the X axis, the position of the shoe in the vertical of Z axis is adjusted either according to a computer program or input from a roller rolling along a template or the like. When the blade finishes its travel in the direction of the X axis, the blade is polished to form a curve in the Z-X plane. In order to define the curve of the turbine blade in the Z-Y plane, the blade must be moved along the Y axis and the position of the shoe in the Z axis must also be adjusted accordingly. Typically, a program or operator instructs the movable platform to move in the Y direction by incremental an amount dependent on the surface finish to be achieved (i.e. move a small amount for a very smooth finish, move a larger amount for a less smooth finish). Whereupon, the platform makes another traverse in the X direction, thereby moving the blade with respect to the shoe while the shoe is instructed to move appropriately in the Z direction.
Since the Z-X profile of the blade is not necessarily constant along the Y axis, the movement of the shoe in the Z direction during each pass of the blade in the X direction will not necessarily be the same as the movement during the previous pass. If a computer program is being used to control the Z movement of the shoe, a new set of points or instructions may be required.
This process is repeated until the turbine blade has fully traversed the shoe in the X and Y directions. The process results in a series of steps approximating curves in the planes parallel to the Z-Y plane (See FIGS. 1 and 3). These steps approximate the curves to be obtained in planes parallel to the Z-Y plane by a number of points between which the shoe polishes a relatively flat surface of the blade (See FIG. 3). Normally, anywhere between 400-600 points are needed to define an approximate curve in a Z-Y plane. The number of points or steps permitted is actually limited by the mechanical devices used. The movable platform and shoe positions are controlled by servo motors which are limited by their acceleration in the number of times they can be started and stopped in a given distance. Thus, when sanding with the belt sander is completed, the surface of the curves in the Z-Y planes are approximate, being a series of flat surfaces rather than a continuously curving surface. Depending on the number of points or steps used, further hand sanding may be required. The hand sanding is quite tedious requiring the use of guillotine gauges and constant reference to a table of measurements.
Moreover, the known methods and apparatuses for polishing turbine blades have no way of incorporating a full rational movement of the turbine blade about the X axis while sanding. Only one side of the turbine blade may be sanded and the blade must then be re-mounted to sand the other side. Thus, the belt sanding methods and apparatuses currently known, do not allow for completely automated production of turbine blades, but require several manual operations including tedious manual finishing in order to smooth out the complex surface of the blade.